(1) Field of the Invention
The present invention relates to maneuvering an underwater vehicle, and more specifically to systems and methods for generating vehicle maneuvering forces from a propulsor.
(2) Description of the Prior Art
Standard torpedoes and Unmanned Undersea Vehicles (UUVs) utilize a single propulsor at the stern coupled with control surfaces to provide the vehicle with necessary forces and moments to offer control. At higher speeds, this combination generally is satisfactory in terms of offering sufficient control. At low speeds, control surface effectiveness is significantly diminished, with the extreme condition being zero forward velocity (e.g., Bollard condition). There are several operations where low speed control is vitally important for UUV mission requirements. These include UUV recovery, station-keeping and synthetic aperture sonar.
Side forces have been generated using thrust vectoring. In this case, the thrust is re-directed off-axis to generate side forces for control. To meet low speed requirements, autonomous research vehicles have utilized tunnel thrusters to offer lateral and vertical control.
The difficulty is that this method is most effective at zero speeds. As the flow velocity is increased, tunnel thruster effectiveness is significantly diminished. It has been shown that tunnel thrusters are only twenty percent effective above five knots. The tunnel thrusters also increase parasitic drag so that maximum velocities are reduced. In addition, tunnel thrusters take up considerable volume that could otherwise be used for energy or payload.
Another concept is referred to as the Haselton bow propulsor, first introduced in the 1960's. In this concept, a pair of propellers, one at the bow and one at the stern, is used in tandem to provide vehicle control. Side forces are generated via cyclic pitch actuation similar to that used for helicopter rotors.
The design utilizes a swash plate so that angle of attack is varied during a single propeller rotation. For example, if maximum and minimum angles of attack are reached at 0° and 180°, the higher thrust force at 0° and lower thrust force at 180° will generate a moment couple. By adding rake and skew to the propeller, it is then possible to generate a substantial side force component.
The disadvantage is that the Haselton bow propulsor concept remains mechanically complex for implementation on undersea vehicles. In addition, placing a propulsor at the bow of the vehicle interferes with forward looking sonar systems.
What are therefore needed are systems and methods for maneuvering an underwater vehicle that are effective at reasonable operating speeds; that do not significantly reduce maximum velocities; and that do not take up considerable volume. Additionally, the systems and methods should be relatively simple to implement without interfering with forward looking sonar.